This present invention relates generally to the calibration of sensors and more particularly to the calibration of orthogonal inertial sensors for use in a navigation system.
Some vehicle navigation systems utilize inertial sensors, such as accelerometers, to propagate the position of the vehicle. In particular, some vehicle navigation systems utilize a suite of nominally orthogonal accelerometers to measure acceleration in each direction to propagate the position of the vehicle.
It is difficult to manufacture a sensor suite comprising a plurality of sensors mounted orthogonally (or some other known relative orientation) with high accuracy. After manufacture, the orthogonality of the sensor suite is tested. The sensor suite is mounted to a precisely formed cube. The cube is then placed on a precisely level surface, sequentially on each face of the cube. At each orthogonal orientation of the sensor suite and cube, readings from each of the sensors are taken. If a cross-coupling of the sensors (deviation from perfectly orthogonal) exceeds a predetermined threshold, that sensor suite is discarded. Typical yield rates for sensor suites with orthogonal accelerometers maybe less than ten percent. At the same time, the processes for trying to achieve precise orthogonality of the plurality of sensors is expensive, despite the low yield.
The present invention provides the ability to accurately use sensor suites with sensors that are not perfectly arranged in the desired orientation. For example, the present invention permits the use of three accelerometers that are mounted at an orientation at some deviation from mutually orthogonal. The cross-coupling of the sensors is first measured and then utilized to generate a transformation matrix. The transformation matrix is then utilized to correct the signals from the sensors and remove the cross-coupling. The present invention also simultaneously compensates for offsets in each of the sensors in the sensor suite.
As a result, lower cost processes can be used to manufacture the sensor suite, while also obtaining higher accuracy. Further, a higher yield is obtained, since sensor suites that previously would have been discarded can now be utilized, with the corrective transformation matrix.
Preferably, the transformation matrix is stored on the sensor suite in some readable form. In this manner, the sensor suite can be sold separately and subsequently installed into a navigation system (for example). A CPU in the navigation system then reads the transformation matrix that corresponds to that sensor suite and utilizes the transformation matrix to correct the data from the sensor suite.